The present invention relates to a catalyst comprising silica and an acid phase containing sulphuric acid and its use in catalytic alkylation of isobutane and/or isopentane by at least one olefin, which allows at least one product, for example dimethylbutanes, trimethylpentanes, trimethylhexanes and trimethylheptanes, to be obtained.
It is known that in order to supply internal combustion engines with spark ignition, and particularly engines with a high compression ratio, it is particularly advantageous to have fuels with high octane ratings, that is to say consisting essentially of heavily branched paraffin hydrocarbons. The alkylation of at least one isoparaffin (isobutane and/or isopentane) by at least one olefin containing 3 to 6 carbon atoms per molecule allows such products to be obtained. This reaction requires the use of very acidic catalysts, particularly for reducing parasitic reactions such as hydride abstraction from the olefin and of polymerization which provide slightly branched hydrocarbons with a low octane rating and unsaturated hydrocarbons, cracking reactions and dismutation reactions.
The existing processes for production of hydrocarbons by alkylation of isobutane by olefins generally use either sulphuric acid or hydrofluoric acid as the catalyst. In these processes the acid catalyst constitutes a liquid phase which is placed in contact with the liquid isobutane/olefin(s) mixture to form an emulsion. These processes are expensive and give rise to significant problems with regard to personal and environmental safety. In order to remedy these problems, different catalytic systems of sulphuric acid and hydrofluoric acid in liquid phase have been sought.
European patent application EP-A-0 539 277 describes a catalyst containing silica and a solid acid phase comprising sulphuric acid, the silica according to said patent application is such that its porous volume is between 0.005 and 1.5 cm.sup.3 per gram and its specific surface is between 0.01 and 1,500 m.sup.2 per gram. Said acid phase can if required contain an additive selected from the group formed by H.sub.3 PO.sub.4, B(OH).sub.3, BF.sub.4 H, FSO.sub.3 H, CF.sub.3 CO.sub.2 H, SbF.sub.5, CF.sub.3 SO.sub.3 H and SO.sub.3.
European patent applications EP-A-0542612 and EP-A-0542620 describe the same type of catalyst, but with different supports: the catalyst support described in EP-A-0542612 contains at least one sulphated oxide and the catalyst support described in EP-A-0542620 is selected from among resins, carbon, zeolites, and oxides such as ZrO.sub.2, T.sub.2 O.sub.2, Al.sub.2 O.sub.3, Fe.sub.2 O.sub.3, HfO.sub.2 and used cracking catalysts.